Acoustic pulses, more particularly ultrasonic pulses, are used mainly for testing and diagnosis during pulse reflection operation. Ultrasonic pulses are usually generated as follows: A system capable of oscillating, e.g. a piezoelectric, magnetostrictive or electric transducer, is excited by a short impulse voltage, e.g. a steep pulse, at one side. The result, depending on whether the transducer is acoustically irradiated or damped, is an acoustic pulse which can be analyzed by the Fourier method into superposed sinusoidal individual oscillations of varying frequency. The frequency spectrum of a thus-produced pulse, which has to be determined and measured, has a maximum at or near the natural frequency of the transducer if the acoustic irradiation is suitable, but comprises a relatively wide region containing marked maxima and minima. This phenomenon is all the more noticeable in proportion to the shortness of the exciting transmitter pulse. FIG. 1 illustrates a typical spectral distribution for the ultrasonic signal of a damped ultrasonic test head (natural frequency of oscillator 4 MHz) when pulses are excited in known manner, e.g. in accordance with FIG. 2.
The behaviour of ultrasonic pulses interacting with the material depends on the frequency of oscillation or on the wavelength .lambda., which is obtained from the relation ##EQU1## In this formula, c is the speed of sound in the medium in question and f is the frequency. The speed of sound c, in the case of longitudinal and transverse waves, is a constant specific to the material. The behaviour of sound in the material in turn determines the result or test values and provides criteria for judging the test results. Consequently, ultrasonic testing and diagnosis are frequently adversely affected if the acoustic pulse has a large band width, since the criteria for evaluating the test data cannot be sufficiently clearly defined.
The wavelength and, consequently, the frequency determine e.g. the geometry of the sonic field, i.e. the short-range field and the divergence angle in the remote radiation field. There is also a relation between the wavelength and the strength of the reflection at inhomogeneities and interfaces between different media. In addition, the attenuation and scattering of sound are dependent on wavelength. These values influence important parameters of ultrasonic testing and diagnosis. In the AVG diagrams, for example, . . . the relation between an idealised equivalent error (a circular reflector perpendicular to the sound incidence), the amplification required for a particular display on the screen of an ultrasonic test device and the distance between the test head and reflector relative to the length of the short-range field, which is dependent on the wavelength. Accordingly, there are considerable limitations to the use of AVG diagrams in describing the behaviour of broad-band transducers. Similar limitations apply to focusing test heads. The focal distance is dependent on wavelength and consequently is not clearly defined in the case of broad-band test heads. A clearly defined focal length can be obtained only by using narrow-band or monochromatically oscillating test heads. The focal length can be varied as required, by varying the frequency of oscillation.
The existence of guided waves (e.g. plate and tubular waves) is also dependent on the geometry of the test-pieces at the frequency of sound. Accordingly, wide-band acoustic irradiation is also unsuitable for producing the last-mentioned waves.
In material testing, cases occur where the reflector signal which is to be detected, e.g. a fault in a weld seam, is disguised or even overlaid by scattered reflections from the surrounding structure. The reflections from the structure become greater in inverse proportion to the ratio of the wavelength to the particle size. They thus become stronger if there are more high-frequency sound components in the transmitted pulse. As a result, the rated frequency of the oscillator has to be reduced, in order to keep the broad frequency band out of the high-frequency region and thus improve the ratio between the reflection of faults and the reflection of the structure. However, when the rated frequency is reduced, it also becomes more difficult to detect small reflections. It would therefore be advantageous to vary the bead width instead of the rated frequency, in order to improve the ratio between reflections of faults and reflections from the structure.
Theoretically, monochromatic oscillation can be obtained by exciting a transducer by a d.c. train of infinite duration. In pulse reflection methods, however, the exciting voltage train must be short, since reflected pulses cannot be received during transmission. If, however, the voltage train is switched on and off, the oscillation is overlaid by building-up and dying-out transient processes, which can be explained on the basis of the mass and spring forces of the transducer system. These building-up and dying-out processes have an effect on the frequency spectrum of the generated pulse which increases with the shortness of the monochromatic a.c. voltage train.